The invention relates to a circuit arrangement for voltage regulation having a voltage divider, which is arranged between a first potential and a reference-ground potential and which has a multiplicity of diodes connected in series, wherein it is possible to tap off an output voltage at a terminal of a diode. The circuit arrangement also has a regulating circuit, to which the output voltage and a reference voltage are applied for the purpose of regulating the first potential on the basis of a comparison of the output voltage with the reference voltage, wherein it is possible to alter the divider ratio by activating or deactivating one or more diodes.
Such circuit arrangements for voltage regulation are used for example in integrated circuit arrangements in which a voltage is generated which is greater than the supply voltage of the integrated circuit. Such voltages are required for example in order to erase memory cells of a non-volatile memory, in particular EEPROM memories.
The problem that arises in this case is in regulating the potential difference between the first potential and the reference-earth potential, the potential difference being referred to as high voltage hereinafter. Since the high voltage lies above the supply voltage, it is not possible directly to measure and to regulate this high voltage. For this reason, voltage dividers are used, so that the measurement and regulation can be effected at a lower voltage level lying below the supply voltage.
Two different types of voltage dividers are usually used. If a precise setting possibility is required for the divider ratio, dividers are constructed from resistor chains. Individual resistors can be bridged in order to set the divider ratio. The fineness of the setting possibility results from the magnitude of the respectively bridged resistor in relation to the total resistance of the divider. However, such dividers have the disadvantage that the area requirement is comparatively large and this therefore constitutes an unfavourable solution from cost standpoints.
A more favourable solution with regard to the area requirement consists in constructing the voltage divider from diodes; in particular, dividers comprising MOS transistors each connected as a diode are known. In order to be able to use such a divider, however, it is a prerequisite that the minimum required setting granularity of the divider is greater than the threshold voltage of the transistors. In this case, the voltage is set by activating or deactivating individual diodes. If a realistic value of approximately 0.6 V is assumed for the threshold voltage of the transistors, the high voltage can only be set in steps of 0.6 V.
In order to be able to refine the setting granularity in the previous solution for realizing a voltage divider, the nominal voltage drop across a divider element must be reduced, so that, by means of the activation or deactivation of the divider element, the total voltage can be altered by a voltage drop of 0.2 V, for example. In such a case, however, diodes or MOS transistors can no longer be used since their threshold voltage is reached at 0.6 V and a voltage divider constructed in such a way is no longer functional below that.